This invention relates to a drive device for driving light emitting elements such as light emitting diodes (LEDs) operated at high voltages, and to an electronic apparatus equipped with such light emitting elements.
Light emitting elements such as LEDs are used not only as display elements themselves but also as backlight sources of a liquid crystal display (LCD). The number of light emitting elements used depends on the form of the display and the amount of light required for the display.
FIG. 4 illustrates a conventional circuit for driving LEDs for use with an electronic apparatus such as a cellular phone. The circuit includes a drive device 30 for driving a display device 40.
The display device 40 has groups of two serially connected LEDs 41 and 42 (the groups referred to as a first light emitting element series), two serially connected LEDs 43 and 44 (the groups referred to as a second light emitting element series), and two serially connected LEDs 45, and 46 (the groups referred to as a third light emitting element series). The numbers of light emitting element series and the LEDs in the respective series are given merely for illustration. The numbers and configurations of the series and LEDs can be determined arbitrarily as needed.
On the other hand, the drive device 30 includes a step-up type switching power supply circuit 31 for stepping up a power supply voltage Vdd (typically 4V) of a lithium battery for example to a higher step-up output voltage Vh. The step-up voltage Vh is fed back as a detection voltage Vdet to a control circuit 32. The control circuit 32 controls the power supply circuit 31 such that the voltage Vh remains constant by comparing the detection voltage Vdet with a reference voltage (not shown).
The step-up voltage Vh is set to 9V say, based on the fact that a white and a blue LED requires about 4V for emission of light. This step-up voltage Vh is applied to the LEDs 41-46 through the pin P31 of the drive device 30 and the pin P41 of the display device 40.
Since LEDs are constant-current elements, drivers 33-35 are usually implemented as constant-current drivers activated by respective constant-currents. Each of the constant-current drivers 33-35 provides a constant current Il when turned ON, irrespective of the number of LEDs in a series, and shuts down the current when turned OFF. The drivers are respectively turned ON or OFF in accordance with respective instruction signals S1-S3 to control associated LEDs 41-46 of the display device 40.
Incidentally, although a constant current Il is provided to the LEDs of a series for emission of light, voltage drop across one LED differs from one LED to another due to the fact that LEDs have production tolerance. As a result, the voltage drop varies in the range of about 3.4V-4.0V for a white LED when the constant current Il is 20 mA.
On the other hand, the constant-current drivers 33-35 are usually implemented in the form of transistor circuits, which are adapted to perform constant-current operations in the active region of the transistors. Therefore, as shown in FIG. 5, in order to place a transistor in its active region, a voltage greater than Vce0 is required across the collector and the emitter. (The voltage will be referred to as transistor voltage.) In FIG. 5, Ic represents collector current of a transistor. If the voltage applied to the transistor is less than the predetermined transistor voltage Vce0, for example Vce2 as shown in FIG. 5, the transistor falls into a saturation region, whereby the transistor cannot maintain its constant current operation any longer. Then, the required constant current Il is not provided to the LED, so that the LED stops emission of light and fails to function as a light-emitting element of the display.
In order to circumvent such condition, the step-up voltage Vh is set to a voltage, for example 9V, that is sufficient for activation of two LEDs each requiring at most 4V, plus the transistor voltage Vce0 and an extra margin.
In actuality, however, the constant-current drivers 33-35 are each impressed with the voltage that amounts to the difference between the step-up voltage Vh and the voltage drop across the associated LEDs. This voltage difference is shown in FIG. 5 as transistor voltage Vce1. The voltage difference turns out to be 2.2V for example when the voltage drop per LED is 3.4V. As the number of the LEDs in the series increases, this voltage difference becomes still larger.
The foregoing discussion on the variation of the light emitting characteristic also holds in a case where a multiplicity of light emitting element series are driven by a step-up voltage. It is necessary then to set the step-up voltage Vh at a higher voltage that takes account of the variations in the characteristics of the multiple series. As a consequence, the current drivers are impressed with higher voltages than necessary.
It is noted that the difference a between the actual transistor voltage Vce1 and the actually required transistor voltage Vce0 results in an energy loss in each of the constant-current drivers 33-35. For this reason, it is necessary to make the constant-current drivers 33-35 large in size, which will lower the power efficiencies of the drive device.
It is, therefore, an object of the invention to provide a drive device for driving light emitting elements, formed of low-voltage ICs and operable with a reduced power loss. This can be attained by forming the drive device such that it always provides a lower voltage than a power supply voltage to the pins to which the light emitting elements are connected, irrespective of the number of the light emitting elements connected. It is another object of the invention to provide an electronic apparatus equipped with such light emitting elements.
It is a further object of the invention to provide a drive device comprising a multiplicity of constant-current drivers for driving multiple groups of serially connected light emitting elements (the groups referred to as light emitting element series), the drive device adapted to automatically control the voltages impressed on the drivers to a predetermined level while performing its normal constant-current operation with a reduce power loss, irrespective of the variations in light emitting characteristic of the light emitting elements. It is a still further object of the invention to provide an electronic apparatus equipped with such light emitting elements.
In accordance with one aspect of the invention, there is provided a drive device for driving a multiplicity of light emitting element series each including at least one light emitting element, the drive device comprising:
a multiplicity of drivers having first ends connected to a multiplicity of terminals to which the light emitting element series are respectively connected, each of the drivers turned ON or OFF in accordance with an instruction signal supplied thereto such that, when turned ON, said driver provides a current to associated one of the light emitting element series for emission of light;
a selection circuit receiving the voltages that are respectively impressed on the drivers, the selection circuit adapted to select the lowest voltage from the voltages and output the lowest voltage as a detection voltage; and
a control circuit for controlling, the drive voltage applied to the light emitting element series by a power supply circuit by comparing the detection voltage with a reference voltage to generate a control signal to the power supply circuit so as to equilibrate the detection voltage with the reference voltage. The light emitting elements may be light emitting diodes.
In accordance with another aspect of the invention, there is provided an electronic apparatus equipped with light emitting elements, the electronic apparatus comprising:
a display device having:
a power supply circuit for converting a given power supply voltage to another output voltage in response to a control signal supplied thereto; and
a multiplicity of light emitting element series each including at least one light emitting element and having a first end connected to the output voltage and a second end connected to associated one of different terminals, and
a drive device having:
a multiplicity of drivers having first ends connected to the different terminals, each of the drivers turned ON or OFF in accordance with an instruction signal supplied thereto such that, when turned ON, the driver provides a current to activate associated one of the light emitting element series for emission of light;
a selection circuit receiving voltages that are respectively impressed on the drivers, the selection circuit adapted to select the lowest voltage from the voltages and output the lowest voltage as a detection voltage; and
a control circuit for outputting the control signal to the power supply circuit so as to equilibrate the detection voltage with the reference voltage by comparing the detection voltage with a reference voltage. The light emitting elements may be light emitting diodes.
In this arrangement, light emitting element series are respectively turned ON or OFF in accordance with the ON-OFF status of the associated drivers. Moreover, the output voltage of the power supply circuit is automatically controlled in such a way that the detection voltage is equilibrated with the low reference voltage for the constant-current drivers to perform their normal constant-current operations. Accordingly, the light emitting elements can be fully energized for emission of light on one hand, and on the other hand the energy loss by the drivers can be minimized, even if the light emitting elements such as LEDs have variations in light emitting characteristic.
The drive device is further provided with a multiplicity of bypass means, each connected in parallel with associated one of the drivers, for providing the light emitting element series with currents that are not sufficient to activate the light emitting element series for emission of light when associated drivers are turned OFF. Hence, the terminals to which the light emitting elements are connected are only impressed with low voltages even when the associated drivers are turned OFF. Therefore, ICs designed to operate only at low voltages (referred to as low-voltage ICs) can be utilized to form the drive device for driving the light emitting element series, irrespective of the voltage required for the light emitting element series to emit light.
The drivers may be constant-current drivers for providing a constant current when they are turned ON. The bypass means may be constant-current sources. When a driver is turned OFF, the current flowing through the associated bypass means can set up a predetermined weak current through it, and hence through the associated light emitting element series. Under this condition, the light emitting element series is maintained in a stable non-luminescent condition.